The standard operating protocols for ISDN (Integrated Services Digital Network) systems severely limit the capability of establishing direct communication between terminal equipment within a given premises. The prescribed physical layer standards applicable to an ISDN customer installation and the call set-up signaling requirements, for example, all but eliminate the ability to use ISDN phones at a single site, such as within a home, in unrestricted intercommunication similar to that normally enjoyed with common analog telephone instruments. As a result, ISDN equipment lacks the usual "extension phone" utility to which users, at least in the United States, have long been accustomed.
As presently structured, ISDN communication systems require that voice messages between terminal equipment units employ the circuit-switched B-channels which are capable of connecting specifically designated terminal units only through at least one local switch via their respective local passive buses, network termination elements, and subscriber lines. Calls between individual terminal unit phones within one premises must be made in the same dedicated manner as those between distant locations, thus there is no means by which a second phone unit at a given location may be included as an "extension" in an ISDN communication. This basic limitation on the utility of ISDN phone systems poses a substantial threat to the widespread acceptability, particularly among domestic users, of this otherwise broadly capable advancement in communications.
The present invention provides a means by which ISDN terminal phone equipment, and ordinary analog phones as well, may be readily and economically employed in an extension phone mode in ISDN voice communication systems.